NEMA Class I transformers

The National Electrical Manufacturers Association (NEMA) has promulgated performance and test requirements for low-voltage designs that can be listed as NEMA Class I, and for such units efficiencies are somewhat higher than those that appear in the literature as standards designs. This article explores the practical economic effects of transformer selection over a range of efficiency levels at and above the NEMA Class I levels. Comparisons of NEMA Class I with standard-efficiency liquid and dry substation transformers and with premium transformers are discussed. Currently there are no specific standards to distinguish premium-efficiency transformers from standard-efficiency transformers. In making the purchasing decision for unit substations, the value of transformer losses is one of the economic factors to be considered.